Note that in general animals which do not
have backbones are called invertebrates.

The main division in the animals we are
likely to meet in our everyday lives is between the group of animals called the
protostomes (such as molluscs and arthropods) and the deuterostomes (to which the vertebrates belong).

Unusual Animal Groups

During the “explosion” of animals during
the Cambrian, many forms existed which seem unrelated to modern animals, such
as this:

Opabinia reconstruction courtesy of Nobu
Tamura

In this story, however, we will concentrate
on those creatures which appeared in the Cambrian and would later become
important in Earth history.

Jellyfish

Cells specialized even more than sponges and tissues(groups of similar cells)
began with the jellyfish family. Jellyfish might have appeared 650 mya. Because
of their soft bodies, they have left few fossil traces.

Nerves and muscles first appear in these
animals, although they had no brain. Their behavior is very simple, responding
automatically to touch. Nerves can carry more information between cells than hormones can. Nerves are the main feature which makes animals different from
plants.

Jellyfish image courtesy of US National
Oceanic and Atmospheric Administration

They belong to the group of animals called
Cnidaria or Coelenterata. This group includes corals, hydras, jellyfish,
Portuguese men-of-war, sea anemones, sea pens, sea whips, and sea fans.

Ancestral Flatworms

Early forms of jellyfish seem to have evolved into various different kinds of worms, one of which was the ancestor of flatworms.
They, in turn, probably evolved into higher animals. Ancestral flatworms may
have first appeared around 570 mya.

Turbellarian flatworm image by Ivy
Livingstone courtesy of BIODIDAC

Here we see the basic plan laid out for the
structure of the body which would be used and modified by all later groups of
animals.

Tissues (first seen in jellyfish) evolved
into organs (parts of the body devoted to doing a specialized job). So, for
example, the flatworms evolved a stomach.
The stomach of the flatworms was just a sac. The worms took food in and send
out waste through their mouths. This was not as efficient as the more advanced
system we are used to, where the digestive system forms a tube with an outlet
separate from the inlet! However it was a big step forward from the jellyfish.
The cells of the stomach specialized in secreting digestive enzymes,
making processing of food more efficient.

Flatworms
crawled forwards and so evolved sensors for smell and touch at their front end.
An organ, the brain, developed at the front to process this information, with
nerves going back to control the flatworm’s body.

Bilateral Symmetry

Their bodies evolved so the two sides were
the same (bilaterally symmetrical). This was quite different from the radially
symmetrical bodies of the jellyfish. This has been the basic design of most the
higher animals. It was probably more efficient for the embryo of the worm to
grow in this way rather than any other way.